The present invention relates to inductive systems for measuring liquid metal level, such as in sodium and sodium-potassium systems. These reactive liquid metals can be used as a coolant in fast flux breeder reactor facilities. The presence and level of the reactive liquid metal must be readily ascertainable, and numerous liquid metal probes are typically incorporated throughout such systems as well as in the coolant systems.
The inductive type liquid metal level sensor generally comprises a bifilar wound transformer which is immersible in the liquid metal containment, although the probe is typically enclosed within a dry thimble or tube, so that it does not actually contact the liquid metal. A constant current power source drives the primary coil and induces an output voltage signal in the secondary coil, which induced signal varies as the level of the liquid metal about the secondary coil. It is thus possible to determine from the variable inductive output voltage the level of the liquid metal in the containment. The resistance of the probe coil varies as the temperature of the coils is varied, and this lessens the accuracy of the probe. With the use of a constant current drive it has been the practice to provide temperature compensation for such an inductive probe by arranging to measure the voltage drop across the primary coil. Since the primary coil resistance for a given temperature is measurable, a calibration curve of primary coil resistance versus temperature characteristic can be worked out experimentally to compensate for the sodium temperature about the primary coil. The liquid metal in the containment systems can be varied from about 300.degree. to 1200.degree.F so that the inductive probe must be operative and accurate over a wide temperature range. The prior art system of temperature compensation had by measuring the resistance change of the primary coil assumes that the inductive probe is in an isothermal environment. The total length of the inductive probe can of course vary over a significant distance, and typical probe coil lengths are from 20 inches to 200 inches. The probe coil is not in fact subject to an isothermal environment until the level of the metal completely covers the active probe length. Thus, serious temperature gradients can exist over the probe length when it is substantially uncovered by the liquid metal. This can lead to considerable inaccuracy for the probe. The reliance of the temperature compensation upon measuring the primary coil voltage drop requires that the wire used in the inductive probe must be tightly controlled with respect to having a linear resistance versus temperature characteristic.